The present invention relates to improvements in loudspeaker accuracy in the field of high fidelity sound reproduction and more particularly to a method and apparatus for achieving such improvements.
The prior art has long been concerned with apparatus and methods of operation for speaker systems in order to accurately reproduce various sounds from an electronic input.
It has been thought by many that the ideal speaker would be a point-source from which all the frequencies being reproduced would emanate. Limitations of technology have prevented the construction of such a driver. In attempting to achieve sufficient acoustical output across the entire audible frequency band, the prior art has generally resorted to the use of a plurality of drivers of differing construction, each type reproducing a specific portion of the frequencies within that band. Moreover, it has been generally recognized that linear reproduction of the audible frequency range is important to achieve realism in the reproduced sound.
More recently, the importance of polarity and phase alignment of the drivers within the system, relative to each other, has been recognized. U.S. Pat. No. 3,824,343 issued July 16, 1974 to Dahlquist identifies and deals with the problem of "time delay distortion" as a barrier to "coherent sound" in multiple driver systems. Dalquist observes that, in a multiple driver system where all drivers are mounted in the same mechanical plane, the leading edge of a single pulse (an electrical test signal which most meaningfully simulates a short duration musical transient) applied to the system input will be reproduced acoustically by the system as a series of pulses that will appear to the listener as a distorted form of the original pulse. This "time delay distortion" has a number of causes. Initially, a lower mass driver-diaphragm, such as a tweeter dome, can be expected to reach more quickly to the pulse than the more massive woofer cone, so its portion of the pulse will arrive at the listener's ear before that of the woofer. In addition, the various elements in a crossover network may effect a group delay on portions of the input signal's spectrum. Thus, while the total energy output of a time delay distorted system may be identical with that of the applied signal, the waveform generated by the system may be substantially different.
In order to compensate for the differing "rise time" of each type of driver, defined as the inherent time lag between impression of a voltage and driver response, Dahlquist contemplated a fixed relative arrangement of the drivers so that the pulse wave form front contribution from each driver would be theoretically synchronized and would reach the ear of the listener at the same time. For this purpose, the higher range speakers were located somewhat further away from the listener than the lower range speakers.
However, because significant distances existed between the drivers of the above system while the location of the drivers relative to each other within the system were fixed, the proper pulse-arrival time alignment of the above system has been found in the present invention to be not only dependent upon freedom of arrangement for the speaker system within a selected listening environment, but more particularly, upon the location of the listening point relative to the speaker system. Speaker systems of the type contemplated by the present invention may be used in many different types of environments including, for example, studios or homes. In many such environments, there exist certain constraints which limit freedom of location for the speaker system. At the same time, a listener, particularly one with a discerning ear, desires to establish the speaker system so that maximum sound coherence is achieved at a predetermined critical listening point. Location of the critical listening point may or may not be determined by choice.
Accordingly, the predetermined relation of components within a speaker system of the type referred to above may not be adaptable to a given listening environment. The result may be a need for compromise between arrangement of the speaker system within the listening environment and the quality of sound reproduced by the system.
There has thus been found to remain a need for a method and apparatus in a speaker system for achieving maximum sound coherency for a plurality of drivers of a speaker system relative to a critical listening point of a selected listening environment while allowing the location of the speaker system and the critical listening point to be dictated at least in part by considerations imposed, for example, by the environment itself and/or by the listener.